bookmarks: Fix thumbnail URL not getting saved
bookmarks: Fix thumbnail URL not getting saved

A cut'n'paste error meant that we saved the description instead of
the thumbnail URL in that field.

jasuarez/grilo-plugins,MikePetullo/grilo-plugins,grilofw/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,GNOME/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,GNOME/grilo-plugins,grilofw/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins